Within the human genome, there is 1 gene for hearing, 3 genes for vision, 12 genes for tasting, and 1,000 genes for smelling. The human nose contains approximately fifty million neuro-receptors connected to ten thousand primary neurons. The latter are in contact with a second layer of neurons linked with the olfactory bulb in the cerebral cortex, which is where odors are recognized. In electronic noses, the neuro-receptors are replaced by a sensor matrix. The interactions between the different gas molecules and the sensors alter certain physical properties of the latter. The overall set of sensor matrix signals yields the “olfactory signature” or “odor pattern” characteristic of a given odor and odor concentration. In the case of the electronic noses, the two neuron layers and the cerebral cortex are replaced by an algorithmic odor recognition and quantification element. The network of artificial neurons is a common solution of this mathematical problem. It is the resemblance of the device with the human olfactory system that led to its being named an “electronic nose”.
An odor is a quality of at least one chemical compound that stimulates the olfactory organ resulting in a sensation. Odor can be defined or quantified by various metrics such as the odor concentration, the odor intensity, the odor character, the odor persistence or the odor hedonic tone.
Odor concentration at the perception threshold is by definition 1 o.u./m3 (odor unit per cubic meter). Odor concentration is expressed as multiples of the perception threshold. By definition [2], the odor unit is the quantity of odorous substance that, evaporated in 1 m3 of odorless neutral gas (CNTP), triggers a physiological odor detection response in 50% of the population. The odor concentration of an odorous gas sample is determined by presenting that sample to a human panel, causing the concentration to vary due to dilution with a neutral gas in order to determine the dilution factor at the perception threshold of 50% of the panel. At that level of dilution the odor concentration, by definition, is 1 o.u./m3. The EN 13725 standard enables, among other things, the determination of the concentration of an odor by means of dynamic olfactometry; since the samples presented to the panelists are not to undergo any pre-treatment, no method for drying the odorous air is used, and the dilution air itself is dry.
The passage from an olfactory signature (the set of sensor matrix responses to an odor of known composition and concentration) to the characterization (recognition and quantification) of the odor is affected by means of a mathematical model. After prior training, the mathematical model will thus correlate an odor (nature and concentration) with its olfactory signature. The mathematical model may take into account parameters other than the sensor responses; for instance, humidity, temperature, air flow or measurement chamber pressure.
There are today various electronic nose (or electronic sensor) technologies to meet the requirements of different industry sectors. The following are among the applications of electronic noses: quality control, environmental monitoring, research and development, the military and security sectors, and the health sector. Electronic noses make it possible to measure odors objectively, precisely, repeatably and continuously.
Different sensor technologies are used for electronic noses, such as MOS (Metal-Oxide Semiconductor), QMB (Quartz Microbalance), IRS (Infra-Red Sensor), CPS (Conducting Polymer Sensor), SAW (Surface Acoustic Wave), OFS (Optical Fiber Sensor), and others. These sensor types have different sensitivity, selectivity, robustness and service life characteristics. The choice and combination of technologies depends primarily on the type of application. Odorous molecule recognition and quantification is made indirectly by measuring changes in some physical properties of the sensors, such as electrical conductivity and the resonance frequency.
However, the sensors used into a sensor network for measuring odors have a limited measurement accuracy and a limited reproducibility of the signals obtained during measurement.